docs: def _compute_intra_event_alpha(self, C, vs30, pga1100):
          """
          Returns the linearised functional relationship between fsite and
          pga1100, determined from the partial derivative defined on equation 17
          on page 148
          """
          alpha = np.zeros_like(vs30, dtype=float)
          idx = vs30 < C['k1']
          if np.any(idx):
              temp1 = (pga1100[idx] +
                       C['c'] * (vs30[idx] / C['k1']) ** C['n']) ** -1.
              temp1 = temp1 - ((pga1100[idx] + C['c']) ** -1.)
              alpha[idx] = C['k2'] * pga1100[idx] * temp1
  
          return alpha